maghemite

Maghemite is a natural ferric iron oxide mineral with the formula gamma-Fe2O3. It is the gamma (γ) polymorph of Fe2O3 and a defective spinel-structured oxide. In maghemite, iron is present exclusively as Fe3+, and cation vacancies on the octahedral sublattice balance the charge. The crystal structure is closely related to magnetite but lacks Fe2+.

Natural occurrence and formation: Maghemite forms by oxidation of magnetite and other iron-bearing minerals in soils, sediments, and rocks. It can accumulate as nanoscale particles in magnetotactic bacteria and in atmospheric dust; it also arises during weathering of iron-bearing minerals.

Physical properties: It is ferrimagnetic and displays magnetic ordering; the material has a spinel crystal structure with a lattice parameter near 8.33 angstroms. At small particle sizes maghemite becomes superparamagnetic and its magnetization is temperature dependent. It is more oxidized and thermodynamically less stable than hematite, and under some conditions it slowly transforms to α-Fe2O3 (hematite).

Applications: Historically, maghemite has been used as a magnetic pigment and in ferrite magnets. In modern science, maghemite nanoparticles are studied for biomedical imaging (MRI contrast), drug delivery, magnetic separation, data storage, and environmental remediation.